- Glass casting
Glass casting is the process by which "cast
glass" objects, either functional or artistic, are produced by allowing molten glass to solidify in a mould (a process called casting). This technique has been used since the Egyptian period. Modern cast glass is formed primarily by two processes, sand casting or kiln casting.
During the Roman period two or more interlocking moulds were used to create blank glass dishes. Glass could be added to the mould either by chip casting, where the mould was filled with chips of glass and then heated to melt the glass, or by pouring molten glass into the mouldStern, E.M., Roman Mould-blown Glass, Rome, Italy: L'Erma di Fretshneidur in association with the Toledo Museum of Art.] . Evidence from
Pompeiisuggests that molten hot glass may have been introduced as early as the mid-first century CE. Blank vessels were then annealed, fixed to lathes and cut and polished on all surfaces to achieve their final shapeGrose, D.F., Early Imperial Roman cast glass: The translucent coloured and colourless fine wares, in Roman Glass: two centuries of art and invention, M. Newby and K. Painter, Editors. 1991, Society of Antiquaries of London: London.] . Pliny the Elderindicates in his "Natural History" (36.193) that lathes were used in the production of most glass of the mid-first century. Italyis believed to have been the source of the majority of early Imperial polychrome cast glass, whereas monochrome cast glasses are more predominant elsewhere in the MediterraneanPrice, J., A survey of the Hellenistic and early Roman vessel glass found on the Unexplored Mansion Site at Knossos in Crete, in Annales du 11e Congres. 1990: Amsterdam.] . Forms produce show clear inspiration from the Roman bronzeand silverindustries, and in the case of carinated bowls and dishes, from the ceramic industryAllen, D., Roman Glass in Britain, ed. J. Dyer. 1998, Princes Risborough, Buckinghamshire: Shire Publications.] . Cast vessel forms became more limited during the late first century, but continued in production into the second or third decade of the second century. Colourless cast bowls were widespread throughout the Roman world in the late first and early second century CE, and may have been produced at more than one centre. Some revival of the casting technique appears in the third or fourth century CE, but appears to have produced relatively small numbers of vessels
Sand castinginvolves the use of hot molten glass poured directly into a preformed mould [Henry Halem (1996) Glass Notes (3rd Edition). Franklin Mills Press] . It is a process similar to casting metal into a mould. The sand mould is typically prepared by using a mixture of clean sand and a small proportion of the water absorbing clay bentonite. Bentonite acts as a binding material. In the process, a small amount of water is added to the sand-bentonite mixture and this is well mixed and sifted before addition to an open topped container. A template is prepared (typically made of wood, or a found object or even a body part such as a hand or fist) which is tightly pressed into the sand to make a clean impression. This impression then forms the mould.
The surface of the mould can be covered in coloured sand to give a surface colour to the sand cast glass object. When the mould preparation is complete hot glass is ladled from the furnace at temperatures of about 1200 C to allow it to freely pour. The hot glass is poured directly into the mould. During the pouring process, glass or compatible objects may be placed to later give the appearance of floating in the solid glass object [ [http://www.lindafraser.com/Courses/course_2.htm Linda R Fraser Sculpture - Sandcast Glass Design Process and Art ] ] . This very immediate and dynamic method was pioneered and perfected in the 1980s by the Swedish artist Bertil Vallien.
Kiln casting involves the preparation of a mould which is often made of plaster or plaster mixtures [Peter Layton (1996) Glass Art. Craftsman House] . A model can be made of wax, wood or metal and after taking a cast of the model, (a process called investment), the model is removed from the mould. One method of forming a mould is by the "
Cire perdue" or " lost wax" method. Using this, a model can be made from wax and after investment the wax can be steamed or burned away in a kiln. The heat resistant mould is then placed in a kiln with a funnel like refractoryopening which is filled with solid glass granules or lumps. The kiln is heated to a high temperature, normally between 800 and 1000 degrees Celsius, and as the glass melts it runs, settling into and filling the mould. Such kiln cast work could be made of very large proportions and was famously brought to a zenith in the glass art world by the famous works of the Czechoslovakian master Stanislav Libensky[Dan Klein (1989) Glass A Contemporary Art. William Collins Sons and Co] .
Pate de verre
"Pate de verre" is another form of kiln casting and literally translated means glass paste [Peter Layton (1996) Glass Art. Craftsman House] [ [http://www.warmglass.com/pate_de_verre.htm Pate De Verre ] ] . In this process, finely crushed glass is mixed with a binding material, such as a mixture of
gum arabicand water, and often with colourants and enamels. The resultant paste is applied to the inner surface of a negative mould forming a coating. After the coated mould is fired at the appropriate temperature the glass is fused creating a hollow object that can have thick or thin walls depending on the thickness of the pate de verre layers.
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